Encapsulation Techniques

Mounting and operation of several detectors in a common vacuum with minimum spacing between consecutive elements makes a real challenge. Encapsulation techniques have been developed to minimize such problems. Placing each encapsulated detector into the vacuum in an individual aluminium cap makes it possible to separate the vacuum of each detector from the cryogenic vacuum shared by all detectors. Encapsulation drastically enhances the germanium detector reliability. This technology is key for many applications, particularly in space, and especially if associated with Ultra High Vacuum. Encapsulated germanium detectors may be easily handled by the users. They may be stored, exchanged or rearranged and be adapted to various applications with different types of cryostats.

A capsule may be regenerated many times and can be thermally annealed in an ordinary oven from neutron or proton radiation damages, without pumping. The life time of such a detector may be estimated to a minimum of seven years without service. But in reality it is much more: The first EUROBALL capsules were delivered in 1992 and are all still in operation. Encapsulated detectors hardness enables a wide application range, such as part of the payload of nacelles, space launchers, etc...

Compact arrays may be designed. The capsules manufactured for EUROBALL offer a typical wall thickness of 0.7 mm with a distance between cap and crystal of only 0.7 mm. These encapsulated detectors may be in contact with one another offering a 3 mm distance between consecutive crystals and a 1.4 mm total aluminium wall thickness.

For better follow-up of scientific progress, some segmented crystals have been encapsulated to offer high granularity, in addition to the previous advantages.

The detector granularity qualifies the number of independent cells constituting this detector. Such detectors allow a significant reduction or gamma ray broadening due to the Doppler effect.

Moreover, the use of internal and external contacts of the crystal provides information on the interacting position:

• Vertically and transversally, by analyzing signals induced by mirror charges.
• Radially by making a pulse shape analysis.

Accurate localization of the interaction points allows not only reduction of the Doppler effect broadening, but also gamma ray tracking.

In addition to these benefits, the segmented detector encapsulation allows the design of complex cryostats, thus signal optimization which is of much interest for pulse shape analysis.

The feasibility of the germanium detector encapsulation was studied in the frame of a collaboration between Mirion, the Jülich research center and the University of Cologne in Germany.